Project Builder Issue

We are under the impression that the kitchen splash back chosen for our new build had been calculated in the contract price. The builder is now telling us this is not the case.
Stapled to the back of the contract, which is signed, dated and shows the contract number, is the colour selection sheet filled in by the consultant, clearly showing the selected item in question.
The consultant is saying that the selection sheets do not form part of the contract, even with the work order stamp clearly visable.
She is asking us to pay an extra $1675.00 if we plan to go ahead with this item, We do not wish to make any further payment to this company other than the contract price. It has taken almost 12 months to get to lock up stage and there are very clear mistakes in the construction.
Do we have any avenue to take to hold the company responsible for the extra cost ?

Thankyou so much for your help Amanda.
After having a conversation with head office in Sydney, including the mention of Fair trading, the managers decided to pay the extra for the kitchen splash back. So a great outcome for us